Unique indexes on link endpoints

[CodesInChaos]

I understand that referencing arbitrary properties of a link source/target is problematic, since they might be in different tables. But the ids of the source/target itself should be available on the link (as foreign key or because the link is embedded into the table). Being able to reference them in a unique index is needed for common scenarios:

1. Array-like multi-links need an unique constraint on (source.id, link.index)
2. Allowing only one link between a pair of nodes needs one on (source.id, target.id)

[elprans]

Array-like multi-links need an unique constraint on (source.id, link.index)

This is already the case by default.

Allowing only one link between a pair of nodes needs one on (source.id, target.id)

Use constraint exclusive for this.

[CodesInChaos]

There is build-in support for ordered links

There isn't, but it's fairly easy to implement via a link property:

type Entry {
    multi link tags -> Tag {
        property order -> int64;
    }
}

That's what I use at the moment, but I'm trying to enforce the invariant that there are no two links originating at the same Entry which have the same order. Which would require a unique index on the (Entry.id, order) tuple.

[elprans]

This should work:

type Entry {
    multi link tags -> Tag {
        property order -> int64;
        constraint exclusive on 
            ((__subject__@source, __subject__@order));
    }
}

[elprans]

So links are not multi-sets by default?

They aren't.

I assume if I want to allow that I have to define a type serving as linker table like in a relational database?

That's correct.

[CodesInChaos]

__subject__@source that's what I was looking for. Is there a similar predefined property for the target?

[elprans]

Is there a similar predefined property for the target?

There is: @target. It seems it's somewhat broken in link constraints though